(Adapted from the Applicant's Abstract) Mammalian cardiac morphogenesis involves a number of fundamental processes which are yet poorly understood at the molecular level, such as the commitment of mesodermic cells to cardiogenic cells, the fusion of cardiogenic mesoderm into a tubular heart, the rightward looping of tubular heart, the formation of valves and septa, the chamber specification, the trabeculation of myocardium, the vasculogenesis, etc. Genes with a spatial and/or temporal pattern of expression in the developing heart are anticipated to play an important role in the formation of a normal heart. Thus, the ultimate goal of this proposal is to identify a subset of spatially and temporally regulated genes during heart development. Towards this goal, the investigators propose to generate a comprehensive catalogue of genes expressed during development of the rat heart, and to conduct Cdna microarray hybridization experiments to identify a subset of spatially and/or temporally regulated genes. Genes identified in this project will be selected on the basis of their differential pattern of expression and/or localization to a chromosomal region known or presumed to encompass a congenital heart disease locus for further functional analysis (Projects 3 and 6) and human genetic analysis (Project 1). More specifically, the investigators propose (1) to generate individually tagged normalized libraries from three regions of the developing rat heart: atrium, ventriculum and atrioventricular canal, obtained from days 12, 14.5 and 17 of embryogenesis, (2) to identify a set of 30,000 unique rat cDNAs from these heart region libraries by an efficient EST- based strategy that the investigators have developed involving serial subtraction of normalized libraries to minimize redundant identification of ESTs while enhancing the representation of rarer transcripts likely to be missed in more random approaches, (3) to construct mouse heart cDNA libraries enriched for full-length clones to facilitate functional and human genetic analyses of selected clones to be performed in Projects 1, 3, and 6, and (4) to generate cDNA microarrays with the unique set of 30,000 rat heart cDNAs identified in this project and conduct hybridization experiments with fluorescently labeled probes derived from mRNAs isolated from different regions of the heart and obtained from different stages of development, thus identifying a subset of genes that are spatially and temporally regulated during heart development.